What is Cast Iron? Its Types, Composition & Applications (PDF)

Introduction

Cast iron is obtained by remelting pig iron with coke and limestone in a furnace called a cupola. It is mainly an alloy of iron and carbon. The carbon content of cast iron varies between 1.7% and 4.5%. It also contains small amounts of silicon, manganese, phosphorus, and sulfur.

Cast Iron is having brittle properties and hence cast iron is not preferable for parts that are subjected to vibration & shocks. Good Casting properties, Low Cost, high compressive strength, excellent wear resistance & machinability property make it a valuable engineering material. The compressive strength of cast iron is much greater than tensile strength. Below is the ultimate strength value of cast iron:

  • Tensile Strength – 100 to 200 MPa
  • Compressive Strength – 400 to 1000 MPa
  • Shear Strength – 120 MPa

Cast Iron: Types

Cast Iron is of the following types:

Grey Cast Iron

Grey cast iron is a type of iron found in castings, known for its grey color and appearance caused by the graphite crushing of the material. What makes Grey cast iron “grey cast iron” is the graphite flake structure that forms during the cooling process from the carbon contained in the component.

The grey color is due to the carbon being in the form of free graphite. It has low tensile strength, high compressive strength, and no ductility. Grey cast iron is having free graphite on its surface and hence it has lubricative properties. For this reason, it is very suitable for areas where a gliding effect is required. Grey cast iron is the result of both the materials used and the processes used to cast the parts.

In other words, the properties of grey cast iron change depending on what kind of material is melted and poured into the mold and what kind of casting method is used. With a powerful microscope, you can see the graphite microstructure that makes grey cast iron very easy to identify.

Castings

Grey cast iron has small black graphite flakes. These flakes cause cracks and make the material look grey. The mechanical properties of grey cast iron are controlled by the size and shape of graphite flakes present in the microstructure. The popularity of grey cast iron (cast iron) components is due to the fact that grey cast iron is one of the cheapest types of iron castings that can be produced and has acceptable ductility, tensile strength, yield strength, and impact strength for most applications cast iron also has an excellent ability to dampen vibrations, making it ideal for machine bases and many enclosure applications.

Grey cast iron has high thermal conductivity and conducts heat more easily through the metal. Grey cast iron parts are often used in machine tools.  

Chemical Composition

Carbon: 3-3.5%, Silicon: 1-2.75%, Manganese: 0.40-1.0%, Phosphorous: 0.15-1%, Sulphur: 0.02-0.15% and remaining percentage is Iron.

Application

Automobile bodies, cylinder blocks, heads, housings, flywheels, pipes and fittings, and agricultural equipment.

White cast Iron
White cast Iron

White Cast Iron

Its white color is due to the fact that it contains no graphite and all the carbon is in the form of carbide (known as cementite), the hardest component of iron. White cast iron has high tensile strength and low compressive strength. Because of its hardness, it cannot be processed with ordinary cutting tools and requires grinding as a molding process. White cast iron can be cast against a mold or made by regulation analysis. This mold is used when products such as car wheels, grain crusher rollers, jaw crusher plates, etc. require a hard, wear-resistant surface.

Chemical Composition

Carbon: 1.75-2.3%, Silicon: 0.85-1.2%, Manganese: Less than 0.40%, Phosphorous: Less than 0.2%, Sulphur: Less than 0.12% and remaining percentage is Iron.

Application

Mining Crusher parts, Roller Crusher, Oil sand Application, Lifting Bars, Slurry Pumps, Cement Mixers, Pipe Fittings.

Malleable Cast Iron
Malleable Cast Iron

Malleable Cast Iron

The malleable iron is a cast iron-carbon alloy that solidifies in the as-cast condition in a graphite-free structure, i.e. total C% is present in the form of cementite (Fe3C). It is less brittle & more ductile as compared to grey cast iron. The tensile strength of the malleable cast iron is usually higher than that of grey cast iron and has excellent machining qualities. Malleable cast iron is preferable for the machine parts in which a degree of accuracy is required.

In order to obtain a malleable iron casting, it the first cast into molds of white cast iron. A suitable heat treatment (i.e., annealing) then separates the bound carbon in white cast iron into graphite nodules.

There are two methods of malleable solid iron particularly whiteheart malleable solid iron and blackheart malleable solid iron.

Whiteheart Process

White iron castings are packed in iron or steel cases surrounded by a mixture of new and used hematite ore. The box is slowly heated to a temperature of 900-950°C and held at this temperature for several days. During this time some of the carbon from the casting is oxidized and the remaining carbon is distributed as small patches throughout the structure. A warming process is followed by a cooling process, which lasts for several days. The result of this heat treatment is a casting that is strong and withstands heat treatment without breakage.

Blackheart Process

The castings used are free of carbon and sulfur. They are packed in a neutral substance like sand and the reduction of sulfur helps speed up the process. The castings are heated to a temperature of 850-900°C and held at this temperature for 3-4 days. In contrast to the White Heart process, this process turns carbon into globules. Castings made with this process are more malleable

Types of Malleable Cast Iron

Black malleable iron

It is obtained after annealing in an inert atmosphere and exhibits black fracture. The microstructure developed in the casting should have a matrix of ferrite with essentially tempered carbon and no flake graphite.

Pearlitic malleable iron

It is obtained after heat treatment and has a homogeneous matrix consisting essentially of pearlite or other transformation products of austenite. Graphite is in the form of lumps of charcoal. The microstructure must not contain flake graphite

White malleable iron

It is obtained after annealing in a decarburizing atmosphere and has a silvery grey fracture surface with a dark grey to the core of black. The microstructure that develops in a cut depends on the size of the cut. Small section castings are mainly ferrite with some perlite.

Chemical Composition

Carbon: 2.16-2.90%, Silicon: 0.90-1.90%, Manganese: 0.15-1.25%, Phosphorous: 0.02-0.15%, Sulphur: 0.02-0.20% and remaining percentage is Iron.

Application

Hubs of wagon wheels, small fittings for railway rolling stock, brake supports, parts of agricultural machinery, pipe fittings, door hinges, and locks.

Nodular Graphite Cast Iron
Nodular Graphite Cast Iron

Nodular Graphite Cast Iron

This type of cast iron is obtained by adding a small amount of magnesium (0.1-0.8%) to the molten metal. The addition of magnesium causes the graphite to take the form of small nodules or spheres instead of the usual angular flakes. It has excellent fluidity, castability, tensile strength, toughness, wear resistance, pressure resistance, weldability, and machinability. Commonly used for castings that require impact resistance, impact resistance, and good machinability.

Chemical Composition

Carbon: 3.60-3.70%, Silicon: 2.20-2.40%, Manganese: Less than 0.3%, Phosphorous: Less than 0.03%, Sulphur: Less than 0.020% and remaining percentage is Iron.

Application

Hydraulic cylinders, cylinder heads, rolling mill rolls, centrifugal casting products, Valves & Fittings, IC Engines, and Steel Mill Rolls.

Chilled Cast Iron
Chilled Cast Iron

Chilled Cast Iron

White cast iron is made by quenching hot metal. Quenching is commonly called quenching, and cast iron produced in this way is called chilled cast iron. All castings are skin cooled by the contact of molten iron with cold sand in the mold. However, in most castings, this hardness penetrates to a very shallow depth (less than 1mm). Castings can be intentionally cooled or inadvertently cooled to a great depth. Targeted cooling is achieved by inserting iron or steel inserts (tremblers) into the mold. When the molten metal contacts the mold, its heat is easily dissipated and a hard surface is formed. Radiators are used on all surfaces of castings that are resistant to wear and friction.

Chemical Composition

Carbon: 3.30-3.50%, Silicon: 2.20-2.50%, Manganese: 0.91%, Nickel: 1.49%, Molybdenum: 0.84%, Chromium: 0.75%, and remaining percentage is Iron.

Application

Air Blast machine, Used for Surface preparation, Ballast.

Mottled Cast Iron
Mottled Cast Iron

Mottled Cast Iron

A product between gray cast iron and white cast iron in composition, color, and general properties. This is obtained in castings with specific wear surfaces that are cooled.

Application

Manhole covers, Pipes, Fire Plugs.

Chemical Composition

Graphite: 1.75%, Combined Carbon: 1.75%, Iron: 93.5%, remaining slag.

Alloy Cast Iron
Alloy Cast Iron

Alloy Cast Iron

Cast Iron containing Silicon, Manganese, Sulphur, and Phosphorous is called Plain Cast Iron.

Alloyed cast iron is created by adding sufficient amounts of alloying elements such as nickel, chromium, molybdenum, copper, and manganese. These alloying elements increase strength and lead to improved properties. Alloyed cast iron has special properties such as increased strength, high wear resistance, corrosion resistance, and heat resistance. Alloy cast iron is widely used in automotive parts such as gears,  cylinders, pistons, piston rings, crankcases, crankshafts, camshafts, sprockets, wheels, pulleys, brake drums, shoes, crusher and grinder parts, etc.

Chemical Composition

Carbon: 0.02-0.03%, Silicon: 0.02-0.10%, Manganese: 0-0.02%, Phosphorous: 0.05-0.25%, and remaining percentage is Iron.

Application

Brake Drums, Disks, and other Automotive applications.

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Atul Singla

Hi ! I 'm Atul. I am PMP certified Mechanical (Piping) Engineer with more than 17 Years of experience. Worked in the field of Plant design for various industries such as refinery, petrochemical & chemical, Fertilizer, gas Processing industries. Developed passion about Piping while working with national & international engineering consultants on diverse projects involving international clients. Developed courses on Piping Engineering to share the knowledge gained after working with many industry experts, through out these years.

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